20' 2x6

[dsullend]

I have a detached 2 car garage. The garage currently has 2x6's spanning the depth of the garage which is just shy of 20'. The area above would make a good attic and is tall enough to stand and walk around. There is no support under the 2x's which causes quite a bit of sag when walked on (I'm no skinny guy). I once saw a TV program (can't remember which) where the contractor attached plywood to the sides of joists using nails and construction adhesive.
Is this an acceptable practice? If so, what size and type plywood should I use? What about nails? Sizes and type? Does the construction adhesive need to be applied in a certain fashion?
I really don't want to replace my current joists if I don't have to.
Thanks
Don

[MountainDon]

Wow, a 20 ft span with 2x6's is a serious overspan. Are they 24 or 16" on center. It's a serious over soan either way.

You'd need an engineer to do the calcs for adding plywood for strengthening, most likely.


There's a joist span calculator here
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

You can play around with different species, grades and sizes to see what would be required if building from scratch to give you some idea of the shortcomings of what you have.

Others may have some ideas....

[Redoverfarm]

If it were limited storage then I would be tempted to use 3/4" ply and sister the joist with another of the same demensions.  But as Don stated you are pushing the limit the way it is.  Adding a ply floor will only increase that load. 

[SouthernTier]

If it were limited storage then I would be tempted to use 3/4" ply and sister the joist with another of the same demensions.  But as Don stated you are pushing the limit the way it is.  Adding a ply floor will only increase that load. 

I think he was implying on using the plywood as the sister member.  Esentially sistering it with a very thin LVL.  A true LVL is stronger than sawn lumber, true, but (a) the plywood would still only extend 8 feet of the span at most, and (b) it's thin.  So I doubt it would work.  It may help if you had an 8-foot span with an undersized joist (e.g. a 2x4 and with far spacing or an expected heavy loading) that you wanted to reinforce, but probably not here.

[Redoverfarm]

SouthernTier I was actually stating to use the plywood glued and screwed to the exisiting joist then followup by sistering a second joist of the same demensions to the previously joist and ply again glue and screwed.  You would end up with a 3&3/4" joist.  You could even get by with 1/2" ply rather than 3/4" and have a finished demension of 3&1/2".  It wouldn't make any difference if the ply was in three pieces across the 20' span as once it is glued and screwed to the previous joist and then the sistered joist it would act as one. I would however alternate the plywood splice every other one so that all the plywood seams would not fall in the same location. I refer to "glued and screwed" but it could be nailed as well after the second joist is sistered with 16d nails.  I think I would make every atempt to rest the sistered joist on the wall plate as the others are.

[John Raabe]

Since most roofs are framed 24" o/c I tried to see what might work with that spacing.

Using the suggested idea of building an in place built-up girder of two 2x6's and 1/2" ply I wanted to know, would it be a safe attic? Girder Info for new members

Not really... I used a light load of 40psf and assumed dsullend could build a solid girder equal to a DF 4x6. Even at a rather bouncy deflection of 1/180 running my beam calc says it isn't enough. . Not a good idea to do this.

[dsullend]

Thanks for the replies.
These are on 16" centers. The front rests on an I beam spanning the front opening and the other walls are concrete block so I "assume" the builder of the garage wasn't planning on using the overhead area for storage. The 2x6's do rest on a sill plate on the concrete bock wall. There currently is no plywood flooring there but that is the plan.
If I sister 2x12's to the 2x6's, would that give me the strength I need? Maybe even add a 1/2" ply in the middle. I can add a center beam with lolly columns, something I was trying to avoid.

[MountainDon]

The AWC calculators indicates that 2x12, Hem Fir #2 can span that using 30 lb live load and L/360 deflection. Add ed to the existing 2x6 it should be able to carry most anything you's put up there, short of heavy machine tools. 2x10 might do the job when added to the 2x6 but that's a guess and a lot depends on what the loads are going to be.

Normal living space floor loads are usually considered to be 40 lb per sq ft, bedrooms and habitable attic spaces at 30 lb